A range finding device for measuring distance based on triangulation by external ray that is suitable for automatic focussing comprises, as FIG. 4 shows, an optical imaging system further comprising a pair of lenses 1.sub.R and 1.sub.L disposed on a front surface of a camera and an integrated semiconductor range finder chip 5 that further comprises a pair of photo-sensor array 2.sub.R and 2.sub.L which converts an image of a subject from the optical imaging system into electric signals; a pair of quantizer circuits 3.sub.R and 3.sub.L which converts the electric signals from the photo-sensor array 2.sub.R and 2.sub.L into digital signals; and a logic circuit block which calculates a distance signal based on the digital signals from the quantizer circuits 3.sub.R and 3.sub.L. The image of the subject T is projected onto the photo-sensor array 2.sub.R and 2.sub.L on the chip 5 through the lenses 1.sub.R and 1.sub.L which are separated by base line length B.
Distance d between the subject and the lenses is given on the basis of triangulation by a following equation: EQU d=.multidot.f.sub.e /(X.sub.1 +X.sub.2)=B.multidot.f.sub.e /X,(1)
where, fe represents distance between the lenses 1.sub.L and 1.sub.R and the photo-sensor array 2.sub.R and 2.sub.L (that is equal to the focussing length of the lenses 1.sub.R and 1.sub.L), X.sub.1 and X.sub.2 represent distances between image spots and standard image spots of the subject T at infinity on the photo-sensor array 2.sub.R and 2.sub.L, and X (=X.sub.1 +X.sub.2) represents relative displacement (phase difference) of the subject image on the photo-sensor array 2.sub.R and 2.sub.L.
The range finding device is integrated into a unit or a module to facilitate mounting the range finding device on a camera manufactured by a camera maker as a range finder user. FIGS. 5 and 6 show a conventional range finder unit for automatic focussing. The conventional range finder unit comprises an optical lens block 1 into that lenses 1.sub.R and 1.sub.L are integrated; a box like optical waveguide case 6 that has a pair of windows 6.sub.R and 6.sub.L and guides rays from the lenses 1.sub.R and 1.sub.L ; the photo-sensor array 2.sub.R and 2.sub.L onto which the rays are projected through a bottom surface of the optical waveguide case 6; and an IC package in that the integrated semiconductor range finder chip 5 is sealed. The range finder unit is divided into three parts. The optical lens block 1 further comprises a protrusion 8.sub.R projecting from the lens 1.sub.R and protrusion 8.sub.R projecting from the lens 1.sub.L. The protrusions 8.sub.R and 8.sub.K are integrated to the lenses 1.sub.R and 1.sub.L respectively. A fitting groove 9 is formed on each of the protrusions 8.sub.R and 8.sub.L. Projections 10 are formed on right and left sides of the upper surface of the optical waveguide case.
In an assembling process of the optical lens block 1 with the optical waveguide case 6, the protrusion 10 is inserted into the fitting groove 9 and then the protrusion 10 is fixed to the fitting groove 9 by injecting a solvent or an adhesive into a gap between the fitting surfaces of the groove 9 and the protrusion 10. When the optical lens block 1 and the optical waveguide case 6 are made of same family of mold resin, the fitting surfaces are bonded together by dissolving the fitting surfaces with a volatile solvent and then evaporating the solvent. An adhesive is used when the optical lens block 1 and the optical waveguide case 6 are made of different material.
The assembling method according to the prior art, however, fixes the optical lens block 1 with the optical waveguide case 6 by a solvent or an adhesive injected into the gaps between the groove 9 of the optical lens block 1 and the protrusion 10 of the optical waveguide case 6 causes poor bonding, bad external appearance and therefore low production yield. If the gap between the fitting surfaces, into which the solvent or the adhesive should be injected, is too narrow when the protrusion 10 is inserted into the groove 9, the solvent or the adhesive is not sucked deep into the gap by capillarity to cause uneven bonding. Then, an excessive solvent or adhesive runs down an outer surface of the case and dissolves the outer surface or solidifies on the outer surface to cause bad external appearance. To the contrary, when the gap is too wide, the solvent or the adhesive is easily sucked into the gap by capillary action. However, parallel orientation of the optical imaging system is distorted and centering of the optical axis of the optical lens block 1 is hardly accomplished because the solvent or the adhesive localizes when the gap is too wide. If one designs the groove 9 and the protrusion 10 with acceptable fitting gap tolerance, the actual gap spacing shows unavoidable dispersion from a product to a product because of the dimensional dispersion caused through the molding process. Thus the conventional range finder unit are subject to poor bonding, bad external appearance and low yield. Further more, productivity is low and production cost is high according to the prior art, because highly mature skilled labor is required for adjusting injection amount of the solvent or the adhesive and selection of the spot where the solvent or the adhesive is injected from.
In view of the foregoing, an object of the present invention is to provide a range finding device that prevents the poor bonding and bad external appearance associated with fixing of optical lens block to the optical waveguide case of range finder unit by providing the range finding device with an injection means that facilitates injection of a solvent or an adhesive.